1. Field of the Invention
The present invention relates to an image processing apparatus, image forming method, and computer program for image forming, especially for performing optical writing with a line head that performs exposure by lighting sublines to thereby form images.
2. Description of the Related Art
A light emitting diode array head (hereinafter referred to as a LEDA head) used in electrophotography image forming apparatuses needs to correct inclination due to its attachment to an image writing apparatus body and a swelling (bowing) of the LEDA head itself. This is referred to as skew correction. In particular, as a technique relating to gradation control when a binary LEDA head is used, known is a technique of lighting one line several times (sublimes emitting) in a sub-scanning direction to thereby light respective sublines at different time to thereby realize a higher gradation level than that obtained by lighting the respective sublines at the same time.
An example of such a technique is disclosed in Japanese Patent No. 4371322, for example. Specifically, Japanese Patent No. 4371322 discloses an image forming apparatus that drives a light source several times to thereby form images. The image forming apparatus includes: a strobe signal for lighting a plurality of light sources arranged in an extension direction of lines at positions of M lines to thereby form gradation images at a plurality of lines; a lookup table that stores values representing the lighting driving patterns of the M lines to be output with respect to one value of input image data which is N-bit gradation data P (M<N); and an exposure pattern switching control unit that reads a value representing the driving pattern corresponding to the input image data from the lookup table to thereby control the light sources. The lookup table stores a set of 2(N−M) values representing the lighting driving patterns of the M lines output with respect to one value of the input image data in a specific order and stores different exposure pattern data values to be allocated to the respective values of the set of 2(N−M) values. The exposure pattern switching control unit includes: a pattern selection signal for performing switching so that the different exposure pattern data values are allocated to adjacent light sources; and a selector that selects a value corresponding to the exposure pattern data value from the set of plural lighting driving pattern values. The value corresponding to the output gradation data of the input image data is a value obtained by dividing the sum of the set of 2(N−M) values corresponding to the input image data divided by the number of values in the set.
However, when writing is performed by lighting sublines, in the gradation control of the related art, it is not possible to realize a higher gradation if skew correction is performed.
Specifically, for example, there is a case in which one line is exposed through four times of subline lighting. Specifically, the first subline is lit for 8 μs, the second for 4 μs, the third for 2 μs, and the fourth for 1 μs; and skew is corrected in each adjacent area by an amount corresponding to one subline (see FIG. 4 described later). In this case, in a skew correction area A (see pixel 0 in FIG. 4), as an example, when forming one pixel, a gradation corresponding to 12 μs can be expressed through lighting of the first and second sublines. However, in a skew correction area B (an area shifted by one subline from the area A: see pixel 1 in FIG. 4), when forming one pixel, since the first subline is lit for 4 μs, the second for 2 μs, the third for 1 μs, and the fourth for 8 μs, it is necessary to light the first and fourth sublines in order to express a gradation level corresponding to 12 μs as like the area A. Thus, the amounts of attached toner are different from the area B (pixel 1 in FIG. 4) to the area A (pixel 0 in FIG. 4), and it is not possible to obtain a desired gradation.